This invention relates to the field of electromagnetic shields and more particularly to housings that shield logic and RF components in portable radios.
The trend in two-way portable radio design has been to use a digital controller to control the functions of the radio, especially in more sophisticated radios such as frequency synthesized and "trunked" radios. Because portable radios are battery operated, low current CMOS microprocessors and logic circuits are usually employed in the controller to extend the radio's operating time between battery charges. It is well known that CMOS microprocessors and logic circuits consume less current when they are operated at lower frequencies, therefore, it is desirable to clock the controller at less than its maximum operating frequency to enhance battery life. Reducing the controller's clock frequency, however, can bring it very close to the radio's receiver IF frequency, causing interference problems. When these two frequencies are close, shielding the radio receiver circuits from the controller becomes difficult because the limited space available in a small portable radio requires that the radio and controller be in close proximity. Similarly, the controller must be shielded from any high power RF transmiter circuits in the radio, while both the radio and controller must also be shielded from the environment.
In a portable radio having controller clock and radio receiver IF frequencies of 450 and 455 KHz respectively, approximately 90 dB of attenuation is required between the controller and the radio to reduce interference to an acceptable level. The use of prior art conductive coatings, however, only provides approximately 20 dB of attentuation. In this application, therefore, a portable radio housing must include an electromagnetic shield that provides at least 90 dB of attenutation between the controller and radio to prevent mutual interference, and it must also provide electromagnetic isolation from the environment.
Because of the size and weight constraints of portable radios, it would be advantageous to build non-shielding functions into the shield. For example, it would be desirable to use the shield as the structural foundation for the radio, to provide acoustic baffling for the radio speaker, and to utilize its mass to sink heat from various power modules in the radio. It would also be advantageous to build shock isolation into the housing as well as utilize the shield to discharge excess static charge which comes into contact with the external controls of the radio.